How to write chemical formulae from ions

Writing the correct formula of an ionic compound is a gateway skill: if the formula is wrong, every equation, mole calculation and reaction that follows is wrong too. The good news is that it comes down to one idea, charges must balance to zero, plus a short list of ions to know. This guide is built around chemical bonding and feeds straight into the mole concept, chemical formula and equation.

The one principle

An ionic compound is electrically neutral overall. So when you combine a positive cation with a negative anion, the total positive charge must exactly cancel the total negative charge. Sodium chloride is NaCl because one Na⁺ (+1) balances one Cl⁻ (−1). Magnesium chloride is MgCl₂ because one Mg²⁺ (+2) needs two Cl⁻ (2 × −1) to balance. Everything else is just applying that rule tidily.

Know your ions first

You cannot write formulae without knowing the common ions and their charges. Memorise these groups; a fuller list lives in the common ions and formulae reference.

• +1 cations: Na⁺, K⁺, Ag⁺, H⁺, NH₄⁺ (ammonium).
• +2 cations: Mg²⁺, Ca²⁺, Ba²⁺, Zn²⁺, Cu²⁺, Fe²⁺, Pb²⁺.
• +3 cations: Al³⁺, Fe³⁺.
• −1 anions: Cl⁻, Br⁻, I⁻, OH⁻ (hydroxide), NO₃⁻ (nitrate).
• −2 anions: O²⁻, S²⁻, SO₄²⁻ (sulfate), CO₃²⁻ (carbonate).
• −3 anions: PO₄³⁻ (phosphate).

Note that some metals form more than one ion. Iron can be Fe²⁺ or Fe³⁺, and copper is usually Cu²⁺. The Roman numeral in the name tells you which: iron(II) means Fe²⁺, iron(III) means Fe³⁺.

The cross-over method, step by step

1. Write the cation and anion side by side with their charges, for example Al³⁺ and O²⁻.
2. Cross over the charge numbers to become the subscripts: the 3 from aluminium becomes the subscript on oxygen, and the 2 from oxygen becomes the subscript on aluminium, giving Al₂O₃.
3. Simplify the subscripts to the smallest whole-number ratio. Mg²⁺ with O²⁻ crosses over to Mg₂O₂, which simplifies to MgO.
4. Add brackets around a polyatomic ion whenever its subscript is 2 or more.
5. Check that the charges balance: total positive equals total negative.

Worked examples

Sodium chloride. Na⁺ and Cl⁻ are +1 and −1, so one each: NaCl.

Aluminium oxide. Al³⁺ and O²⁻. Cross over: two aluminium, three oxygen. Al₂O₃. Check: 2 × (+3) = +6 and 3 × (−2) = −6. Balanced.

Calcium hydroxide. Ca²⁺ and OH⁻. You need two hydroxide ions to balance the +2. Because there are two of a polyatomic ion, use brackets: Ca(OH)₂.

Aluminium sulfate. Al³⁺ and SO₄²⁻. Cross over: two aluminium, three sulfate. Sulfate is polyatomic and there are three of it, so bracket it: Al₂(SO₄)₃. Check: 2 × (+3) = +6 and 3 × (−2) = −6. Balanced.

Ammonium sulfate. NH₄⁺ and SO₄²⁻. Two ammonium balance one sulfate, and ammonium is polyatomic with a subscript of 2, so: (NH₄)₂SO₄.

Iron(III) nitrate. Fe³⁺ and NO₃⁻. Three nitrate ions balance the +3, and nitrate is polyatomic: Fe(NO₃)₃.

When to use brackets, and when not to

Brackets go around a polyatomic ion only when you need more than one of it. Calcium hydroxide is Ca(OH)₂, but sodium hydroxide is simply NaOH, one hydroxide needs no bracket. Copper(II) carbonate is CuCO₃, again no bracket, because one carbonate balances one Cu²⁺. A missing or misplaced bracket changes the formula, so treat it as part of the answer, not decoration.

A quick sanity check

After writing any formula, add up the charges. For Fe₂(SO₄)₃: two Fe³⁺ give +6, three SO₄²⁻ give −6, total zero, correct. If your total is not zero, the subscripts are wrong. This one check catches most mistakes before they spread into an equation. You can see these ions combining in the formation of an ionic compound.

Common mistakes to avoid

• Forgetting a metal’s variable charge, writing FeCl₂ when the question says iron(III), which is FeCl₃.
• Leaving out brackets, turning Ca(OH)₂ into the meaningless CaOH₂.
• Not simplifying, writing Mg₂O₂ instead of MgO.
• Changing the charge on an ion to make it fit; the charges are fixed, only the subscripts change.

Practise until it is second nature

Write ten formulae a day from a mixed list of cations and anions, always finishing with the charge check. Once the twenty or so common ions are memorised, the cross-over method makes every formula mechanical, which frees your attention for the harder parts of an equation. If ion charges or brackets keep letting you down, that is a quick fix with a teacher, our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial; see how it works if you would like your formulae drilled against exam questions.